1. Field of the Invention
The present invention is directed to a miniature electromagnet and relay, and more particularly to an electromagnet assembly for the relay with reduced length and thickness.
2. Description of the Prior Art
Japanese Utility Model early publication [Kokai] No. 1-168941 proposes a miniature electromagnetic relay which is particularly designed to accomplish a compact structure of reduced thickness by the use of an U-shaped flat magnetic plate having opposed yoke legs in combination with an armature extending between the opposed yoke legs. As shown in FIG. 1, the prior art relay includes an electromagnet unit comprising the U-shaped flat magnetic plate 11A of a magnetic material having a core 12A and a pair of first and second yoke legs 13A and 14A depending from the opposite ends of the core 12A. A coil bobbin 20A is fitted around the core 12A to carry an excitation coil 30A. The armature 40A, which is also of U-shaped configuration with a pivot end 41A and a pole end 44A, bridges between the first and second yoke legs 13A and 14A in parallel with the core 12A. The pivot end 41A is kept in constant magnetic coupling with the first yoke leg 13A and is at the same time pivotally supported adjacent to the first yoke leg 13A such that the armature 40A can pivot about the pivot end 41A within a horizontal plane parallel to the core 12A between a set position where the pole end 44A is attracted to the second yoke leg 14A in response to the energization of the excitation coil 30A and a reset position where the pole end 44A is spaced away from the second yoke leg 14A in response to the deenergization of the excitation coil 30A. The armature 40A carries a cam 46A for actuating a movable contact for contact closing and opening. This structure permits disposing the armature 40A together with the first and second yoke legs 13A and 14A within a coil thickness of the coil bobbin 20A and permits the armature 40A to pivot within the coil thickness, whereby successfully reducing the, overall thickness of the electromagnet structure to as little as the coil thickness. However, the second yoke leg 14A itself adds an extra lengthwise dimension L1 to the length of an overall length L0 of the electromagnet, thereby increasing the length of the armature 40A. Further, the armature 40A is required to have an enlarged surface area at the pole end 44A in such a manner as to overlap the second yoke leg 14A to an increased extent, i.e, to have a magnetic gap of increased opposing surface area between the pole end 44A and the second yoke leg 14A for generating a correspondingly strong attraction force at the magnetic gap. Consequently, the armature 40A has to be made to have an increased equivalent weight because of the elongated effective length from the pivot end 41A to the pole end 44A and also because of the enlarged pole end 44A, thereby suffering from a correspondingly lowered armature attraction force at the magnetic gap which might frequently result in unstable and unreliable armature operation. Also, since the second yoke leg 14A in the form of the plate adds an extra dimension L1 to the overall length L0 of the electromagnet, there is a certain limitation as to the length Lc of the excitation coil 20A within the overall length L0 of the electromagnet, thereby limiting a number of coil turns and therefore limiting the electromagnet force obtained in the electromagnet structure of the limited overall length L0, which is also the hindrance to increasing a armature attraction force within the electromagnet structure of the limited lengthwise dimension.